James Robertson, the managing director of Step Two Designs, developed a"6x2" approach for intranet development.
He proposes that you work in 6 month chunks, planning on what you can deliver in the next six months. Any ideas that cannot be met in the six month time-frame are pegged into the following six month period.
I also really like his idea of designating versions of your intranet. (I think we are currently planning for version 5.0).
Read more in his August KM Column: 6x2: a new approach to planning or his book 6x2 methodology for intranets.